It is generally the case that electronic devices tend to steadily decrease in size as technology progresses. For example, portable computers from a few years ago tended to be an inch or more in thickness, whereas newer portable computers are significantly thinner. In fact, with the advent of the MacBook Air and iPad, it is now possible for consumers to purchase portable computers that are significantly less than an inch thick.
However, whilst devices such as computers have decreased in size, the electrical plugs that are typically provided as part of the power supply for such computers have changed little in size over the years. The ultimate illustration of this is the three-pin plug that is used in countries such as the UK (inter alia). This plug is roughly 1.75 inches thick at its thickest point (from the rear of the plug to the tip of the earth pin), or in other word three times thicker than an iPad, for example.
Clearly it would be advantageous if the thickness of the UK plug, in particular, could be reduced, and a number of companies have launched plugs which do indeed reduce the size of the traditional three pin plug.
For example Thinplug Ltd has developed and is now marketing a plug called the ThinPlug which comprises an earth pin that can be folded back into the body of the plug. As shown on their website, www.thinplug.com (see also UK Patent No. 2436899), the earth pin of the ThinPlug pivots about an arm that is rotabable relative to the body of the plug between a stowed position where the earth pin is folded away and an operative position where the earth pin is extended.
Whilst the ThinPlug is undoubtedly significantly thinner than traditional three-pin plugs, British Standards (in particular BS1363-1) dictate that a plug with an earth pin that is accessible in use (i.e. when the plug is plugged into a socket), can only be used in applications where an electrical connection to earth through the earth pin is not required. The effect of this is that the ThinPlug is only suitable for applications where the earth pin is a dummy pin used only to retract the protective gates that normally obstruct access to the live and neutral contacts of a three pin socket.
Another previously proposed arrangement that also seeks to address the aforementioned plug thickness problem is disclosed in UK Patent No. 2381669. This patent discloses a plug that comprises an earth pin which is mounted to an arm, the arm being hinged to the main body of the plug so that it can be moved about an axis parallel to the live and neutral pins from a stowed position to an operative position where the live, earth and neutral pins are arranged in a triangle. The plug also includes a flash plate that is hinged to the plug main body and must be pivoted upwards before the plug can be inserted into a socket. The flash plate has a cut-out which allows the plate to fit around the earth pin when the earth pin is in the operative position.
Whilst the plug disclosed in this patent is also substantially thinner than a conventional plug, the fact that the arm carrying the earth pin is hinged to the plug main body means that it would be problematic to electrically couple the earth pin to earth via the hinge. Furthermore, as the earth pin is accessible via the aforementioned flash-plate cut-out when the plug is inserted in a socket, if the earth pin were used for any purpose other than as a dummy earth pin, then the plug would probably not pass British Standards. As a consequence, the plug disclosed in this document (in a similar manner to the aforementioned ThinPlug) is limited to applications where the earth pin is a dummy earth pin.
Another potential problem with this arrangement is that the hinge attaching the arm to the plug body is exposed during use it forms a point of mechanical weakness. Specifically, if the hinge were to fail whilst the plug was inserted in a socket (for example following an impact with the plug), then access could potentially be had to the live/neutral pins and/or to the fuse. In fact, investigations have tended to suggest that the inherent mechanical weaknesses of this design are such that the plug would only be able to pass the so-called “Barrel test” of British Standard 1363-1 if it were manufactured from specialist materials—the cost of which would render the plug economically unviable.
One other plug that has addressed the aforementioned thickness problem is marketed by a company called SlimPlug and is disclosed in GB2436465 and GB2417141. In general terms, the SlimPlug plug has pins that can pivot between a stowed position and a deployed position, and a button that a user can operate to release the pins, thereby enabling them to be rotated into or out of the plug main body. In this arrangement as access cannot be had to the pins when the plug is in a socket, the plug is suitable for applications where the earth is not a dummy pin, but provides an electrical connection to earth. However, whilst the plug with the pins retracted is significantly thinner than a conventional plug, it is still in the region of an inch thick—or in other words roughly twice as thick as an iPad.
Aspects of the arrangements described herein have been devised with the foregoing problems in mind.